ABSTRACT Retaining HIV-infected persons in medical care is essential to maximize the potential of antiretroviral therapy (ART) to improve individual health outcomes and prevent HIV transmission. Many persons living with HIV (PLWH) who are poorly engaged in care have substance use disorders. Even with case management support and outreach from clinic and health department staff to help patients access HIV care, some persons with substance use disorders do not or cannot engage in HIV care, at least as it is traditionally organized in the U.S. A novel low-threshold incentivized care (LTIC) HIV clinic in Seattle-King County, WA is designed to engage the hardest-to-reach PWLH by making HIV treatment easier to access and incentivizing adherence to medical visits and ART. The goals of the proposed project are to estimate the effectiveness and potential population reach of the LTIC strategy and to understand the mechanism of the intervention if indeed it is effective. Aim 1 of this project is to estimate the effect of the LTIC intervention on patients? HIV outcomes with a retrospective study of the first 50 patients in the Seattle LTIC clinic. We will assess patients? care engagement and viral suppression during the 6 months after LTIC enrollment compared to the 6 months prior to enrollment and compared to the same outcomes in a contemporary matched cohort of patients enrolled in traditional HIV care. The results of this study will allow us to design a prospective trial with sufficient statistical power. Aim 2 of this project is to identify essential components of the LTIC intervention and understand patient perspectives on the relative importance of each component by conducting individual, semi-structured qualitative interviews with 20 LTIC patients. These interviews will inform our plans to optimize the LTIC intervention and adapt it to other sites. Aim 3 of this project is to estimate the size of the target population for the LTIC strategy in Seattle. To estimate the number of viremic HIV-diagnosed persons in Seattle who use methamphetamine, crack-cocaine, or heroin, we will use data from the CDC-funded Medical Monitoring Project, the CFAR Network of Integrated Clinical Systems, and public health partner services data. To estimate how many substance-using PLWH are likely to remain viremic after outreach to relink patients to traditional HIV care, we will use data from our health department and clinic-based HIV care relinkage programs in Seattle. Together, these studies will inform future study of the LTIC intervention, a novel and potentially cost-effective strategy to increase viral suppression among PLWH who have substance use disorders.